This invention relates to a torch assembly for use in a vapor axial deposition (VAD) process in forming optical fiber glass preforms.
In the fabrication of optical fibers, it is customary to create a preform, which is a long glass rod having a central core and which is, in effect, a magnified or enlarged version of the optical fiber to be drawn therefrom. The preform consists of an inner core and an external cladding having an index of refraction profile that reproduces the index profile of the drawn fiber.
There are three major processes for making preforms, the modified chemical vapor deposition (MCVD) process, the outside vapor deposition (OVD) process, and the vapor axial deposition (VAD) process. All of these processes utilize vapor deposition, in which a xe2x80x9csootxe2x80x9d is deposited on the surface of a starting object such as the interior surface of a starter tube or a starting or target rod to form a glassy layer on the surface.
In the VAD process, silica vapors flowing through a heating torch deposit the desired silica particle layers on the rod. As the sooty particles build up to a desired diameter, the target rod is moved upward to make room for further growth, while the torch or torches are fixed in position. When the build-up or deposition is complete, the rod is removed and the resulting preform is sintered or consolidated into a solid preform.
Although the VAD process is widely used, in a production milieu, certain problems arise which prevent complete satisfaction with the method, more particularly, the apparatus. Thus, the glass torches, which must be precisely set for satisfactory and reproducible results, tend, over an extended period of use to become leaky, unstable, and not very consistent in producing repeatable results. For example, the various gases and vapors fed to the torch are generally transmitted through plastic tubing to Teflon fittings which, in turn, connect to the glass torch nipples. These Teflon fittings tend to loosen up over extended periods of use, which can, and often does, give rise to leakage, thereby destroying the calibrated delivery of the gases and vapors to the torch. The tendency to loosen is due, at least in part, to temperature fluctuations and also to lack of strain relief in the plastic delivery tubing. Lack of strain relief is a very important problem, since even small mechanical stress on the tubing can loosen the Teflon to glass interface and cause leaks. For example, during routine machine maintenance, the mechanic can accidentally push the tubing and thereby loosen the interface. A further problem, which can actually be more serious in that it can shut down the production line, is that heretofore the mounting of the tubular glass torch to the adjustable mounts, made necessary for precise positioning of the torch, can cause cracking or breaking of the glass of the torch, necessity replacement thereof, which can cause as much as a week""s delay in production. The glass tube of the torch is, generally, clamped to the adjustment stages using a V-block type clamp. By nature, the glass torch is extremely delicate as well as expensive. In use, this type of clamping technique, if too tight a clamp can cause cracking or breakage of the glass and too loose a clamp results in process instability, thus a certain amount of operator skill is required in positioning and clamping the torch.
The present invention is a torch for use in, for example, the VAD process and includes a mounting assembly therefor which obviates the drawbacks of the prior art assemblies as discussed in the foregoing.
In a preferred embodiment of the invention, the glass torch comprises an outer main tubular body within which may be a plurality of coaxial glass tubular bodies of diminishing size for creating a plurality of gas and/or vapor delivery passages. Such nested nozzles are shown, for example, in U.S. Pat. No. 4,627,866 of Kanamori et al., and in accordance with the invention each of the concentric delivery passages is attached to Teflon fittings connected to glass nipples formed on the nested nozzles. The main tubular glass body is mounted within a metal or other rigid and non-fragile type material torch clamp tube and has an outer diameter less than the inside diameter of the torch clamp tube so that there is clearance therebetween. Within the torch clamp tube, at each end thereof, is a tapered or round ring or ferrule made of somewhat resilient and high temperature material, for example, Teflon, whose inner diameter is, preferably, a slip fit over the outer main glass torch tube, and which serve to hold the main glass tube in suspension within the clamp tube. Each end of the clamp tube has external threads formed thereon and first and second compression nuts are screwed thereon. The compression nuts, when tightened, bear against the ferrules to tend to push them longitudinally relative to the clamp tube. The tapered or round, where an O-ring is used, surface of the ferrules bears against the inner surface of the main tube ends causing them to exert an increasing clamping force on the main nozzle body, thereby affixing it concentrically in position within the clamp tube. The stress thus induced in the glass main nozzle of the torch is evenly distributed over an area around the entire torch periphery and is well below critical levels for glass. It has been found that hand tightening the compression nuts is more than adequate to secure the torch firmly within the glass clamp tube. Thus, the danger of too much stress being applied to the torch main tube even during temperature fluctuations, which could produce cracking or breaking, is no longer a consideration. The torch clamp tube, and not the thin glass tube, is used to mount the torch to the adjustment stages. The mounting arrangement of the invention, therefore, is more robust and safe, and provides several other benefits in addition to those just discussed. For example, the ferrule is made of elastic and resilient material such as Viton or Teflon, or other high temperature plastic and acts as a temperature compensating member during expansion and contraction of the glass and the metal. It also acts as a heat isolator allowing the gases within the torch to stay warm; acts as a vibration and shock isolator/damper, thereby protecting the glass torch; and it provides easy rotary and linear (in/out) coarse torch alignment adjustment.
Another feature of the preferred embodiment of the invention is directed to the mounting system for the torch and the torch clamp tube to the adjustment stages and to inclusion in the assembly of the gas delivery system. The mounting system comprises a torch support rail having tracks or slots machined or extruded therein holding strain relief clamps for the gas delivery tubing. The tracks in the support rail and the clamps make possible both longitudinal and lateral positioning of the strain relief clamps and, therefor, the gas delivery tubing. Each of the concentric torch nozzles has a nipple to which is attached a temperature compensating Teflon fitting, to which a gas delivery tube is connected. Adjustment of the position of each strain relieving clamp relieves its associated tubing of any kinks, sharp bends, or other stress inducing positions, and the Teflon fittings and glass nipples are isolated from the rest of the system by the clamping action of strain relieving clamps.
At one end (the large torch end) of the rail member is mounted to a support block or bottom clamp having an arcuate section for receiving the torch clamp tube which is clamped therein by a top clamp half, thus holding the torch clamp tube. A second mounting clamp grips the torch clamp tube and is mounted to a mounting stage for coarse longitudinal adjustment of the torch position. At the opposite or rear end of the rail member is a rear mounting tube plate which has a bore therein through which a single, axially aligned supply tube passes to a Teflon fitting and a longitudinally extending torch glass nipple. The bore has a strain relieving clamp surrounding it for clamping the supply tube in position. As will be apparent hereinafter, the strain relieving clamps for the supply tubes are angled in a manner to position the tubes optimally as they lead into the corresponding Teflon fittings. Thus the tubes are fixed in position without inducing any stress on the tubing and fittings, so that leakage at the fittings does not result as the assembly is moved or during machine maintenance.
As a consequence of the unique assembly construction, uniform clamping pressure is placed on the glass torch body, thereby lessening the occurrence of cracking or breaking; the assembly is robust and stable and much more immune to the effects of continued use, and of temperature fluctuations; the torch mount is in effect temperature compensated, and all glass to pipe fittings are virtually leak proof; and no load is placed upon the glass nipples, thereby eliminating the risk of cracking or torch damage.